The system of a screw compressor is roughly classified into two different types, that is, an oil-flooded screw compressor in which a lubricating oil is supplied to a rotor compression chamber and an oil-free screw compressor in which no oil is supplied to the rotor compression chamber.
FIG. 4 shows an oil-flooded screw compressor 100. In this oil-flooded screw compressor 100, a pair of male and female screw rotors (not shown) inside a compressor main unit 102 are driven by a motor 101 so that a process gas from a process-gas supply source 103 is compressed and supplied to a supply end 105 through an oil recovery device 104. The oil, separated in the oil recovery device 104, is supplied to bearings (not shown) and a compression chamber (not shown) of the compressor main unit 102 through an oil cooler 106, a pump 107 and a filter 108. In the oil-flooded screw compressor 100, the lubricating oil is supplied to the compression chamber (not shown) of a rotor (not shown) so that the process gas is cooled, deriving a merit that a high compression ratio can be achieved by compression at the first stage.
In the case when a gas containing much heavy hydrocarbon gas, such as propane, butane and hexane, is used as the process gas, the heavy hydrocarbon gas is dissolved in the lubricating oil, causing a reduction in the viscosity of the lubricating oil and damages to the bearing. Moreover, in the case when the heavy hydrocarbon gas is compressed to cause a pressure increase, although the gas is liquefied at a low temperature state, it is not liquefied at a high temperature state. In order to increase a discharge temperature so as not to liquefy the heavy hydrocarbon gas, it is necessary to increase the temperature of the lubricating oil supplied to the compression chamber (not shown). However, the increase in the temperature causes a reduction in viscosity of the lubricating oil and the subsequent damages to the bearing. In contrast, in the case when the temperature of the lubricating oil is lowered and the discharge temperature is also lowered so as to ensure the viscosity of the lubricating oil, the heavy hydrocarbon gas is condensed inside the oil recovery device 104 to cause a rise of the liquid level, resulting in a problem of scattering of the lubricating oil toward the succeeding flow.
FIG. 5 shows an oil-free screw compressor 120. In this oil-free screw compressor 120, screw rotors 123 and 124 inside a compressor main unit 122 are driven by a motor 121 so that a process gas from a process-gas supply source 125 is compressed and supplied to a supply end 126. On the other hand, a lubricating oil inside an oil tank 127 is supplied to bearings 130 through an oil pump 128 and a filter 129, and is then returned to the tank by gravity. In the oil-free screw compressor 120, since no oil is used for lubricating the screw rotors 123 and 124 and maintaining an air-tightness of the compression chambers (not shown), shaft-sealing seals 133 at four positions are required for separating the compression chamber (not shown) from oil injection portions 132 of the bearings 130 and timing gears 131. As the seals, those which use carbon, or gas seals may be used. Since the shaft-sealing portions 133 at four positions are required, reliability against seal leakage is low, and the compressor becomes expensive.